The invention is related to the field of laser technology, and in particular to an optimized cascaded Raman fiber-based laser source for high efficiency mid-infrared spectral generation.
Light sources spanning mid-IR wavelengths are the subject of intense interest for applications such as semiconductor processing, coherent X-ray generation, chemical sensing and cancer detection. To this end, sources spanning mid-IR wavelengths have been developed in the form of quantum-cascade lasers, rare-earth doped fiber-gain media, and fiber-based supercontinuum sources.
It has been demonstrated that at high powers, substantial levels of Raman gain can be achieved through the use of conventional silica fibers to produce significant spectral red-shifts through spontaneous cascaded Raman (CR) processes. Knowing this, the possibility of constructing simple and efficient Raman-based sources and gain-blocks for long-wavelength spectral generation seems promising. However, one's ability to continually red-shift through CR does suffer some practical limitations. Spectral broadening and pulse distortions due to both spontaneous Raman emission and four-wave mixing (FWM) generally limit the efficiency of this process.
For instance, at high powers, anomalous dispersion and Kerr nonlinearities can give rise to modulation instabilities (MI), which result in a significant spectral broadening and pulse distortion, unnecessarily stifling the efficiency of the cascaded Raman process. For this reason, normal dispersion fibers are more desirable for controlled and maximally efficient CR at the nanosecond time-scale.
Unfortunately, silica fibers generally possess anomalous dispersion from 1.3 μms to 2.4 μm due to the presence of a strong mid-IR absorption resonances.